The present invention relates to apparatus and a method for processing a distorted signal to reduce the effects of the distortion and in particular to such circuitry which processes a signal that has been subject to clipping distortion to recover a replica of the signal without the clipping distortion.
Clipping distortion is a common phenomenon in many types of analog signal processing. It may occur, for example, in an analog voltage amplifier when the potential of the input signal and the gain of the amplifier result in an output signal which would have voltage values that are either above the positive supply voltage or below the negative supply voltage. In these instances, the level of the output signal is limited or "clipped" at the level of the supply.
Another method by which clipping distortion may occur is in a laser modulation system. An example of how this may occur is shown in FIG. 3. This Figure shows the transfer function 310 of a typical semiconductor laser diode as the relationship between input current, I(t), and output optical power, P(t). The input current to the laser diode is biased at a current Ib. Signal variation about Ib causes the diode to produce proportional amounts of optical power. There is a threshold input current, Ith, below which the diode will cease to emit light. As shown in the Figure, when the input current I(t) falls below this threshold, the power output of the laser diode falls below zero and the light signal P(t) is clipped.
One common technique which is used to prevent clipping distortion is to carefully control the amplitude of the input signal or the gain of the system to prevent clipping. One such technique is described in U.S. Pat. No. 5,168,526 to Orban, entitled DISTORTION-CANCELLATION CIRCUIT FOR AUDIO PEAK LIMITING. In the system described in this patent, a voltage controlled attenuator is used to reduce the gain of the system by an amount proportional to a low-pass filtered difference between the input signal and the output signal.
Another technique for restoring a clipped signal is disclosed in U.S. Pat. No. 3,510,640 to Voelker, Jr., entitled METHOD AND APPARATUS FOR INTERPOLATION AND CONVERSION OF SIGNALS SPECIFIED BY REAL AND COMPLEX ZEROS and in a paper by Voelcker et al. entitled "Clipping and Signal Determinism: Two Algorithms Requiring Validation" IEEE Transactions on Communication, June, 1973, pp 738-744. These references describe an iterative technique by which, a clipped signal is analyzed to locate its real and complex zero-crossing points and this data is used to reconstruct the signal irrespective of its actual amplitude values.
A third technique is disclosed in two articles by Logan, Jr. entitled "Signals Designed for Recovery After Clipping--I. Localization of Infinite Products" and "Signals Designed for Recovery After Clipping--II. Fourier Transform Theory of Recovery" ATT Bell Laboratories Technical Journal, vol. 63, no. 2, Feb. 1984 pp 261-306. These articles describe an iterative method by which a class of clipped signals may be recovered using zero-crossing points and either localized exponential functions or Fourier transformation.
These techniques may use significant amounts of memory because they sample the input signal to determine the base points from which interpolation is to be performed and store these samples for analysis. In addition, because of the extensive mathematical analysis performed by some of these techniques, they may not be applicable to real-time signal processing. This is especially true for high-frequency operations, such as recovering information from a radio-frequency (RF) modulated signal produced by a semiconductor laser diode.